Vapor or particle detection device

ABSTRACT

A VAPOR, SMOKE OR PARTICLE DETECTION DEVICE OF THE TYPE COMPRISING A BLACK-WALLED CHAMBER CONTAINING A LIGHT SOURCE AND A PHOTOCONDUCTIVE CELL, ARRANGED TO DETECT LIGHT FROM THE SOURCE WHEN REFLECTED FROM PARTICLES PRESENT IN THE CHAMBER, AND INCLUDING A SELF-BALANCING CONTROL CIRCUIT FOR ACTUATING AN ALARM WHEN THE DETECTION SIGNAL FROM THE PHOTOCONDUCTIVE CELL EXCEEDS A GIVE LEVEL. THE SENSITIVITY OR THRESHOLD LEVEL OF THE ALARM CONTROL CIRCUIT IS MAINTAINED CONSTANT DESPITE WIDE VARIATIONS IN THE APPLIED OPERATING VOLTAGE BY MEANS OF A VOLTAGE DIVIDING BRIDGE AR-   RRANGEMENT USING AN INSULATED-GATE FIELD EFFECT TRANSISTOR FOR OPERATING THE GATE OF A CONTROLLED RECTIFIER WHICH CONTROLS CURRENT FLOW TO THE ALARM DEVICE. ALSO PROVIDED IN COMBINATION ARE MEANS FOR INTERMITTENTLY OPERATING THE LIGHT SOURCE FOR LONGER BATTERY LIFE, A HIGH TEMPERATURE DETECTION ELEMENT AND A FAIL-SAFE MECHANISM FOR SIGNALLING WHEN THE DEVICE IS INOPERATIVE.

Jan. G s wHn- ETAL VAPQR OR PARTICLE DETECTION DEVICE 1 Filed Oct. 29.. 1968 3 Sheets-Sheet 1 INVENTOR5 GEORGE 5, WHITE BY RICHARD E. LEWiS LICOLN M. ZONN Arrouey Jan. 12, 1971 G. 5. WHITE T AL 3,555,532 VAPOR OR PARTICLE DETECTION DEVICE Filed 001;. 29, 1968 3 Sheets-Sheet 2 Jan. 12, 1971 s w n- ETAL I VAPOR 0R PARTICLE DETECTION DEVICE Filed Oct; 29, 1968 3 Sheets-Sheet 3 T 1' A 28 I/6 @l N SELF- L (mm/00$ LIGHT Sou/26E United States Patent O Int. Cl. G08h 17/10 U.S. Cl. 340-237 3 Claims ABSTRACT OF THE DISCLOSURE A vapor, smoke or particle detection device of the type comprising a black-walled chamber containing a light source and a photoconductive cell, arranged to detect light from the source when reflected from particles present in the chamber, and including a self-balancing control circuit for actuating an alarm when the detection signal from the photoconductive cell exceeds a given level. The sensitivity or threshold level of the alarm control circuit is maintained constant despite wide variations in the applied operating voltage by means of a voltage dividing bridge arrangement using an insulated-gate field effect transistor for operating the gate of a controlled rectifier which controls current flow to the alarm device. Also provided in combination are means for intermittently operating the light source for longer battery life, a high temperature detection element and a fail-safe mechanism for signalling when the device is inoperative.

CROSS-REFERENCE This application is a continuation-in-part of an application for patent filed by the present inventors on improvements in or relating to vapor or particles detection devices, Ser. No. 585,782, filed Oct. 11, 1966, now Pat. No. 3,474,435, issued Oct. 21, 1969.

BACKGROUND OF THE INVENTION The present invention relates generally to fire and smoke detection and alarm systems and more particularly to a constant sensitivity alarm device for monitoring the presence of smoke, vapor or other particles in the ambient atmosphere.

A variety of electronic devices have been developed for monitoring the level of smoke, vapor or other particles present in a room or enclosure to detect and call attention to abnormal conditions which may result from fires, explosions or other dangerous occurrences. The circuitry associated with such devices has generally been complex and expensive as well as susceptible to failures which are not readily made known to those relying on the operativeness of the apparatus. The introduction of transistorized circuits has alleviated a number of the operating problems but many difliculties still remain such as regulating the sensitivity of the alarm circuit, increasing the life of the components, avoiding component failure at the critical moment and detecting fires which produce heat but a minimum of smoke.

The present invention is intended to provide a smoke detecting device of increased reliability which may also be made heat sensitive and whose sensitivity is maintained constant over long periods of use. In addition, attentiongetting alarm signals and a fail-safe feature may be included.

SUMMARY OF THE INVENTION The present invention embodies a smoke detection device comprising a chamber containing a light source and Patented Jan. 12, 1971 a photoconductive cell, which is arranged to detect light from the source only when reflected from particles presentin the chamber, and includes a self-balancing control circuit for actuating an alarm. The self-balancing control circuit, which is actuated when the light detection signal from the photoconductive cell exceeds a given level, comprises a voltage dividing bridge arrangement using an insulated-gate field effect transistor for operating the gate of a controlled rectifier which controls current flow to the alarm device. By virtue of this bridge arrangement the sensitivity or threshold level of the alarm control circuit is maintained constant despite wide variations in the applied operating voltage. Other features incorporated in the device are circuits for intermittently operating the light source and the alarm device, alternate power sources, and a fail-safe mechanism for actuating the alarm when the light source is burned out, along with sensitivity adjusting means, and means for detecting the presence of high temperatures in the absence: of smoke.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in elevation of the vapor or particle detection chamber of the present invention;

FIG. 2 is a cross-sectional view of the chamber taken along the line 2--2 of FIG. 1;

FIG. 3 is a schematic diagram of the operating circuitry of one form of detection device disclosed in the parent application;

FIG. 4 is a schematic diagram of the operating circuitry of another form of detection device disclosed in the parent application;

FIG. 5 is a schematic diagram of the operating circuitry of the present invention using an amplification circuit containing a self-balancing alarm control circuit and including high temperature sensitive and fail-safe features;

FIG. 6 is a schematic diagram of the operating circuitry of a modified form of the present invention;

FIG. 7 is a schematic diagram of the operating circuitry of another modified form of the present invention; and

FIG. 8 is a schematic diagram of the operating circuitry of a form of the present invention including the use of a speaker in the alarm system.

DETAILED DESCRIPTION The smoke detecting portion of the device, shown in FIGS. 1 and 2, comprises a vapor or particle detection chamber, indicated generally by the numeral 10, consisting of an open-ended cylinder 12 and two end plates 14, 16 which fit loosely over the ends of the cylinder 12 and have inturned peripheral flanges 15, 17. Spring tabs 18, extending from the ends of the cylinder 12 hold the end plates 14 and 16 thereon and maintain the end plates flanges 15 and 17 outwardly spaced from the walls of the cylinder 12. All the inner surfaces of the chamber 10 are painted dull black so as to be nonrefiective.

As best seen in FIG. 2, a light source 20, such as a bulb or glow lamp, is mounted in a tube 22 extending radially from the wall of the cylinder 12. This tube 22 also contains a lens 24 which focuses the light 21 from the bulb 20 into a similar tube 26 located directly across the cylinder 12 on the opposite side, the light 21 being focused into a small area on an end cap 28 for the tube 26. The two tubes 22 and 26 minimize the dispersion of light from the bulb 20 into the main portion of the cylinder 12 thus tending to prevent reflection of light from the cylinder walls.

Extending at right angles to the previously described tubes is a third tube 30, which protrudes inwardly through the wall of the cylinder 12 while also projecting outwardly beyond the wall. In the outer portion of the third tube 30 there is located a photoconductive cell 32, which may be a cadmium sulfide cell, and a lens 34 to focus light reflected from any vapor or particles present within the cylindrical chamber onto the cell 32. The field of view of the lens 34 is greater at the portion of the cylinder wall diametrically opposite the third tube 30 than at the point where the light beam 21 emanating from the bulb crosses the field of view of the lens 34. The inner portion of the tube is cut on a bias so that the open end is directed away from the light bulb 20. Moreover, part of the open end of the tube 30 is blocked by a lip so that indirect rays from the end cap 28 will not be reflected onto the photoconductive cell 32.

Smoke detection occurs generally as follows. Smoke or other particles in the room containing the device will enter the chamber 10 through apertures 18a between the spring tabs 18 of the cylinder 12 and the end plates 14 and 16. When the smoke or particles drift into the beam of light 21, the light is reflected by them onto the photoconductive cell 32, causing the cell 32 to produce a signal indicating the presence of the light reflecting particles. When the signal reaches a given threshold level, appropriate circuitry, connected to the cell 32 through leads 33, is activated and accordingly operates an alarm device. The associated operating circuitry comprises the present invention.

Accordingly, improved electrical circuits are provided for operating the light bulb 20, amplifying the signal from the photoconductive cell 32, and actuating the alarm device. These circuits are shown in FIGS. 3 through 8 with the circuits of FIGS. 3 and 4 being substantially those shown in the previously cited copending application Ser. No. 585,782, of the present applicants, of which this application is a continuation-in-part.

The circuit disclosed in FIG. 3 shows the general arrangement of all the composite circuits and particularly consists of a relaxation oscillator circuit, at the left side of the figure, for controlling the light source, such as glow lamp 20a; a gating and amplification circuit, at the right side, for increasing the strength of the signals sent by the photoconductive cell 32 to an alarm horn 36; and a circuit, at the upper center, for resetting the gating circuit to its original standby condition.

In the oscillator portion of the circuit during normal operation, capacitor 42 is slowly charged by current from battery 84, which current passes through resistor 40 until the firing voltage of unijunction transistor 38 is reached. At that point capacitor 42 is rapidly discharged through transistor 38 and resistor 44, producing a current pulse through resistor 44 and a voltage pulse across resistor 40. Resistor 43 serves to transmit bias current to the base of the transistor 38. A portion of the current pulse passing through transistor 38 is divided off and passes through resistor 46 to the cathode gate of a controlled rectifier 48. The rectifier 48- is triggered to operation by this pulse and acts as a switching element to supply the correct current and voltage to intermittently light the glow lamp 20a. The pulse rate may be set at approximately one pulse per second by determining the appropriate circuit parameters.

The oscillator circuit in this embodiment is particularly intended for use with a long life glow lamp and the actual lighting of the glow lamp 20a is accomplished as follows. While capacitor 42 is being charged, capacitor 50 is simultaneously being charged through resistor 52. During the charging period of time, the controlled rectifier 48 is in the off state. When a current pulse arrives at the cathode gate, the rectifier 48 rapidly switches to the on state and discharges capacitor 50 through the transformer 54, which, by mutual inductance, charges the glow lamp 20a until its firing voltage is reached. When capacitor 50 is fully discharged, transformer 54 produces a small reverse'charge on the capacitor 50, which in turn results in a reverse voltage across the controlled rectifier 48, switching the rectifier to its off state. The entire charging cycle is then resumed. The lamp 20a is thus intermittently lit at a rate which may be set or adjusted by varying the circuit parameters of the oscillator.

When the pulses of light from the glow lamp 20a are reflected by smoke or vapor, or the like, they fall on the photoconductive cell 32. The photoconductive cell 32 is included in an amplifying and alarm operating circuit. The light falling on cell 32 reduces its resistance and an indicative current is thereby caused to flow to the base of a current-amplifying transistor 56, which is biased by a current through resistors 58, 60, and 62. The bias current determines the sensitivity of transistor 56 to the current from the photoconductive cell 32 and can be set at a level for maximum sensitivity by adjusting the variable resistor 58. Also, it will be seen that the pulsed emitter current when transistor 56 is conducting flows through resistors 64 and 66 and sets up a pulsed potential at capacitor 68, which results in a higher input impedance for current pulses passing through the input circuit to the base of transistor 56. Resistor 70 limits the maximum current through transistor 56 while capacitor 72 allows for a higher peak current to be transferred at the moment the current pulse arrives at the base. Thus, when the signal fom photoconductive cell 32 reaches a given threshold value, transistor 56 is gated to conduction and provides an amplified current pulse which appears at the cathode gate of controlled rectifier 74. The controlled rectifier 74 is switched rapidly to its on state when amplified current from transistor 56 arrives at its cathode gate. Conduction of controlled rectifier 74 then causes current to flow through the alarm device or horn 36 producing an appropriate alarm.

Since the horn 36 is operated by a rapid make and break device that causes an intermittent current flow through the horn 36, the current to the anode of the controlled rectifier 74 is pulsed at a higher frequency than the gating current being transmitted fro-m the photoconductive cell 32. The controlled rectifier 74 would then ordinarily be shut oif immediately after each gating. To prevent this a transistor 76 is provided to supply a holding current for the controlled rectifier 74 through resistor 78, so that the rectifier 74 remains in its on state during the period when the current through the horn 36 diminishes to zero. Resistor '80 allows a bias current to pass to the base of transistor 76, so that the latter remains in a saturated state and produces the required holding current. The horn 36 is thus caused to operate and produce an alarm when photocell 32 is sufficiently activated to gate amplifier transistor 56. The alarm will ordinarily operate until controlled rectifier 74 shuts off.

The alarm may be made intermittent in its operation, however, by the provision of a reset circuit including capacitor 82 which is connected between controlled rectifier 48 and transistor 76. When rectifier 48 provides a pulse of current, its anode voltage changes rapidly to below ground potential, resulting in a very sharp negative current through capacitor 82 to the base of transistor 76. This turns off the holding current supplied by transistor 76 to controlled rectifier 74. If the horn 36 is drawing no current at this point, controlled rectifier 74 switches rapidly to its off state, where it remains until another current pulse is supplied to its cathode gate. If the horn 36 is conducting the alarm continues until the current shut off in both transistor 76 and horn 36 is coincident. As long as photocell 32 continues to supply a signal the alarm will be reactivated. Thus, horn 36 may be made to produce a pulsing alarm to better attract attention.

.The battery 84, which acts as the power supply for the circuit, need only be of moderate size to operate the device for many months, if the proper values are selected for the components of the circuit to minimize the current to lamp 20a, as the lamp 20a is only lit intermittently. Also, the use of a lOTlg life glow lamp will further increase the reliability of the detection device.

A more simplified modification of the above-described circuitry is shown in FIG. 4. Here a multivibrator or square-wave oscillator is used to provide intermittent operation of a conventional filament-type light bulb 20b. The oscillator circuit comprises two transistors 86 and 94 with base bias resistors 92 and 96 and coupling capacitors 90 and 98. An output resistor .100 is connected to the emitters of both transistors. Transistor 86 in the left leg of the circuit has a resistance 88 connected to its collector and the bulb 20b is connected to the collector of transistor 94 in the right leg of the circuit. When the right leg is conducting the bulb 20b will light and when conduction switches to the left leg through transistor 86, the bulb 2012 will shut off. As before, the values of the circuit components can be adjusted to provide the desired flashing rate.

The amplification circuit of FIG. 4 is substantially the same as that described in FIG. 3 with the exception that the holding current producing transistor 76 with the resetting circuit are omitted. A shunting capacitor 79 for the horn 36 is substituted in their place.

While the two operating circuits shown in FIGS. 3 and 4 provide comparatively simple and inexpensive, but yet affective, alarm systems, upon consideration it will be seen that the sensitivity of these devices will be effected by variations in the supply voltage. This is a particularly important factor when the devices are operated by batteries over a long period of time since as the battery voltage diminishes, the biasing in the amplification circuit will change. The reliability of the apparatus thus decreases with time, which is, of course, undesirable.

A more simplified circuit for operating the detector device, which circuit is of improved reliability and maintains a constant sensitivity, despite wide variations in the supply voltage, is shown in FIG. 5. This circuit may be used with a battery or alternatively with a commercial 1l5-volt, 60-cycle power supply and has a heat sensitive feature, for responding to high temperatures unaccompanied by smoke, as well as a fail-safe mechanism for signalling when the light bulb 20b has broken down.

As seen in FIG. 5, no oscillator or multivibrator is used to pulse the light source 20b, but rather the source 20b is constantly lit, being operated directly by the battery '84 or the alternate l-volt power supply 101 through a transformer 102. Diode 150 and resistor 151 protect the battery 84 from the alternating current from transformer 102. The current to the photoconductive cell 32 is half-wave rectified by a diode 11-8, with a protective filter including capacitor 112 and resistor 113. The light sensitive cell 32 itself is connected in a voltage dividing bridge arrangement along with an insulatedgate field effect transistor 104. The bridge arrangement includes photoconductive cell 32, which is connected to the base of the field effect transistor 104 along with bias resistor 106, and resistors 108 and 110 also connected to field effect transistor 104. The output terminal or drain of the field effect transistor 104 is connected through resistor 108 to the gate of a controlled rectifier 114 which controls the flow of current through the alarm horn 36.

. and the low resistance load across the bridge, the leading edge in rise and fall time in combination with the wave shape pulse height will produce a pulse height which can be controlled by the resistance of the photoconductive cell 32. So, when the photoconductive cell 32 receives light, being biased part way up from the dark, high resistance region, it will respond much more quickly to a given change in light versus resistance. This response will add to the superimposed A.C. wave component permitting it to reach a threshold value with respect to the field effecttransistor 104. Thus, both DC. and A.C. biasing are used on the photoconductive cell 32 to produce 6 a controlled pulse of sufiicient height to trigger the controlled rectifier 114 as follows.

When light falls on the photoconductive cell 32 the increased conduction will reach a threshold value gating on the normally off field effect transistor 104. Conduction of the transistor 104, in turn, triggers the controlled rectifier 114. Current is thus caused to flow through the horn 36 resulting in an alarm signal. The current through the horn 36 is half-wave rectified by the controlled rectifier 114, which is continuously reset by the alternating voltage from the transformer 102.

It will -be seen that, in addition to the improved biasing, by virtue of the inherent automatic gain control properties of the field effect transistor 104 this control circuit is self-balancing for a wide variation of changes in applied voltage, as sometimes occurs with a commercial power supply or battery source, without disturbing its sensitivity. This constant sensitivity improvement is accomplished while reducing the number of circuit components required.

The circuit may also be made sensitive to high temperatures unaccompanied by smoke by the insertion of a heat sensitive switch 116 connected in parallel with the controlled rectifier 114. The switch 116 may be of the bimetal type and when actuated permits a steady current to flow through the horn 36 providing a signal in contrast with that produced by the rectified current through the controlled rectifier 114.

The light source 20b in this arrangement is steadily lit and may be utilized as an indicator to show that the unit is operating. In the event that the light bulb 20b fails, a relay 119 is provided which is thereby energized and closes the switch 120 placing a battery 122 in circuit with the horn 36. Thus, a steady signal will result until the battery 122 is d sconnected or the defective bulb is replaced.

A modified circuit substantially combining the circuits of FIGS. 4 and 5 is shown in FIG. 6 and provides the features and advantages of both. Alternate power sources may again be used comprising: a commercial supply 101 and transformer 102, and (2) a [battery 84. These alternate power sources are, connected in parallel with the diode 150 and resistor 151 in the battery leg which causes the stand-by battery 84 to be charged when the commercial power source 101 is in use.

The multivibrator or square wave oscillator shown in FIG. 4 is used to intermittently operate the light source 20b. An additional improvement in this ocillator circuit is the inclusion of a Zener diode 124 connected across the lamp 201). The Zener diode. 124 closely regulates the voltage across the lamp 20b so that the intensity of illumination will not vary with the supply voltage. Thus, both the lamp operating and the amplification circuits are voltage balanced.

The field effect transistor 104, controlled rectifier 114, heat sensitive switch 116 and the bridge resistors 106, 108, and '110 are connected in the same manner as shown in FIG. 5 to operate the alarm horn 36. An additional transistor 126 with a base bias resistor 127 is connected across the horn 36 through a resistor 128. This transistor 126 is effective to intermittently shunt the horn 36 to cut off the alarm 50 that in the presence of smoke, the horn 36 operates intermittently at the frequency of the flashing light source 20]). An additional shunting leg including the capacitor 130 and resistor 132 is provided as a bypass filter for occasional voltage peaks which may appear on the commercial supply or the stand-by battery 84.

Another modification of the smoke detector is shown in FIG. 7. In this modification the glow lamp 20a and incandescent bulb 20b are replaced by a light source 20c of the radioactive type, such as a commercially available self-luminous tritium activated light source or the like. Lights of this type are sold under the registered trademark Betalight by Canrad Precision Industries of New York.

The self-luminous light source 20c requires no external power supply and may be used, for example, with the circuit shown in FIG. 6 by adding an additional field effect transistor stage of amplication in the gate circuit to the controlled rectifier 114. The two stages then comprise field efiect transistors 104a and 104b with their associated bias resistors 108a, 110a and 108b, 1101). The power requirements of the device are thus reduced by the use of the self-contained light source 20c while the reliability is increased.

If it is desired to operate the device solely by battery, the improved photocell biasing may be achieved by including a unijunction oscillator 170 along with filtering capacitor 171 in the circuit in lieu of the commercial power supply 101. The unijunction oscillator 170 introduces an additional biasing pulse on the supply voltage from battery 84 rendering the photocell 32 and amplifier arrangement more sensitive to impinging light.

The alarm control section of the circuit also includes a variable resistor 138 which may be used to vary the bias on the gate of the field effect transistor 104a to adjust the sensitivity of the device. By varying the resistance 138 the user can thus reduce the sensitivity of the device during those occasions when the normal concentration of smoke will increase such as from cigarette cigarette smoking during a meeting or party being held in the room containing the detector.

The modification of the invention shown in FIG. 8 is intended for use with a speaker 146 instead of a horn.

Here, the multivibrator used for controlling the lamp 2% is the same as that used in FIG. 6 with the addition of a diode 136 connected between the base and emitter of transistor 86 to act as a voltage limiting device preventing the transistor 86 from receiving a large negative pulse.

The amplification portion of the circuit is modified so that the controlled rectifier 114 operates an oscillator 140 which drives the speaker 146 providing the alarm signal. The oscillator circuit 140 comprises amplifying transistors 141 and 144 in combination with unijunction transistor 142 which determines the timing of the oscillator output. Resistors 141a, 141b, 1410, 142a, 142b, 144a and 144b, along with capacitors 143, 145 and 147 provide the proper biasing for the oscillator 140. It will be see-n that either the speaker 146 or the horn 36 may be replaced by a suitable electronic means which is capable of emitting a high intensity audio signal.

An improved smoke and heat detection device is thus provided which requires a minimum of operating power, which may be made self-balancing for a wide variation of change in applied voltage, and which may be simply modified to provide various types of alarm signals. The unit may be operated from a commercial power source or a battery and is provided with a fail-safe feature and heat sensing means. All of the improved features are accomplished with comparatively simplified and inexpensive circuitry using a minimum of components.

What is claimed is:

1. A device for detecting the presence of suspended light reflective particles in a light transmitting fluid medium comprising:

(a) means defining a chamber having nonreflecting internal surfaces and a passage through said chamber open to the flow of said medium;

(b) means including a light source for directing a beam of light across said chamber;

(c) photosensitive means exposed to the interior of the chamber and out of alignment with said beam so that it is subject to illumination only by light refiected from particles suspended in said beam, said photosensitive means having an electrical characteristic varying as a function of the intensity of the light impinging thereon;

((1) power supply means including a pair of power supply lines;

(e) a first electrical connection between said power supply lines including the photosensitive means and a resistor connected in series; and

(f) an alarm to be actuated when the density of particles in said medium exceeds a predetermined value;

wherein the improvement comprises:

(g) said power supply means, including a direct current power supply and an alternating current supply connected in parallel across said power lines;

(h) a half-wave rectifier connected in series with one of said power lines; and

(i) a filter connected across said power lines on the opposite side of said rectifier from the power supply means, said filter being effective to bypass high frequency currents;

(j) said power supply means being effective to bias said photocell with a unidirectional potential on which an alternating component is superimposed to increase periodically the sensitivity of the device;

(k) a normally off field effect transistor having a gate connected to the common junction of said photosensitive means and said resistor;

(1) second and third electrical connections between the source and drain electrodes of the field effect transistor and the respective power supply lines;

(m) a voltage divider including two resistors connected in series in one of said second and third connections, said photosensitive means being effective when the intensity of the light impinging thereon exceeds a predetermined level to switch said transistor on and thereby to produce a substantial current in said voltage divider;

(n) a normally off controlled rectifier having a gate,

an anode and a cathode;

(o) a connection between said gate and a junction in said voltage divider, said connection cooperating with said gate to switch said controlled rectifier on in response to said current flow in the voltage divider; and

(p) an energizing circuit for said alarm responsive to current flow through the anode and cathode of said controlled rectifier.

2. A device for detecting the presence of suspended light reflective particles in a light transmitting fluid medium comprising:

(a) means defining a chamber having nonreflecting internal surfaces and a passage through said chamber open to the flow of said medium;

(b) means including a light source for directing a beam of light across said chamber;

(c) photosensitive means exposed to the interior. of the chamber and out of alignment with said beam so that it is subject to illumination only by light refiected from particles suspended in said beam, said photosensitive means having an electrical characteristic varying as a function of the intensity of the light impinging thereon;

(d) power supply means including a pair of power supply lines;

(e) a first electrical connection between said power supply lines including the photosensitive means and a resistor connected in series; and

(f) an alarm to be actuated when the density of particles in said medium exceeds a predetermined value;

wherein the improvement comprises:

(g) a normally off field effect transistor having a gate connected to the common junction of said photosensitive means and said resistor;

(h) second and third electrical connections between the source and drain electrodes of the field effect transistor and the respective power supply lines;

(i) a voltage divider including two resistors connected in series in one of said second and third connections,

9 said photosensitive means being efiective when the intensity of the light impinging thereon exceeds a predetermined level to switch said transistor on and thereby to produce a substantial current in said voltage divider;

(j) a normally off controlled rectifier having a gate,

an anode and a cathode;

(k) a connection between said gate and a junction in said voltage divider, said connection cooperating with said gate to switch said controlling rectifier on in response to said current flow in the voltage di- Wider;

(1) said light source being a self-luminous radioactive source; and

(In) said connection between the gate of the controlled rectifier and the junction in the voltage divider includes:

(1) a second normally off field efiect transistor having a gate connected to the junction in the voltage divider;

(2) a second voltage divider including two resistors connected in series between one of the source and drain electrodes of the second field effect transistor and one of the power supply lines; and

(3) a connection between the gate of the con trolled rectifier and the junction between the two resistors in the second voltage divider; and

(n) an energizing circuit for said alarm responsive to current flow through the anode and cathode of the controlled rectifier.

3. A device for detecting the presence of suspended light reflective particles in a light transmitting fluid medium comprising:

(a) means defining a chamber having nonreflecting internal surfaces and a passage through said chamber open to the flow of said medium;

(b) means including a light source for directing a beam of light across said chamber;

(c) photosensitive means exposed to the interior of the chamber and out of alignment with said beam so that it is subject to illumination only by light reflected from particles suspended in said beam, said photosensitive means having an electrical characteristic varying as a function of the intensity of the light impinging thereon;

(d) power supply means including a pair of power supply lines;

10 (e) a first electrical connection between said power supply lines including the photosensitive means and a resistor connected in series; and (f) an alarm to be actuated when the density of particles in said medium exceeds. a predetermined value;

5 (g) amplifying means responsive to the output of the photosensitive means and controlling said alarm; wherein the improvement comprises:

(h) said power supply means, including a direct current power supply and an alternating current supply connected in parallel across said power lines; (i) a half-wave rectifier connected in series with one of said power lines; and (i) a filter connected across said power lines on the opposite side of said rectifier from the power supply means, said filter being eifective to bypass high frequency currents; (k) said power supply means being eifective to bias said photocell with a unidirectional potential on which an alternating component is superimposed to increase periodically the sensitivity of the device.

References Cited UNITED STATES PATENTS 2,119,326 5/1938 Grant, Jr. 340-237(S)UM 2,509,497 5/1950 Hesson 340237 2,620,385 12/1952 Grant, Jr. 340237(S)X 2,988,708 6/1961 Schmidt 340-384EX 3,255,441 6/1966 Goodwin et a1. 340237(S)X 3,283,299 11/1966 Savind 340-384EX 3,382,417 5/1968 Armstrong 317148.5X 3,460,267 8/1969 Lorene 307118X 3,469,112 9/1969 Hands et a1. 328151X 2,500,547 3/1950 Kalmus et a1. 2 1250-217X 3,320,430 5/ 1967 Gorman 307--218X 3,345,536 10/1967 Atkins et. a1. 315-83 3,383,670 5/ 1968 Roberts 3-40--237 3,392,352 7/1968 White 307--251 3,401,274 9/ 1968 Marsh 331113X THOMAS B. HABECKER, Primary Examiner D. MYER, Assistant Examiner US. Cl. X.R. 

